My machine is a very similar size, and also with 1605 driven 1-1 from a NEMA23/3Nm. I get around 5000mm/min as well, although I haven't really tried winding it up much more as that is about as much as the long axis will take anyway. That was my design goal in any case as, as you say, that is about the corner speed of those motors when they start to lose torque. Can't remember the acceleration I'm using - I'll try to check next time I fire it up.

Brian's law of motor current tuning is - "keep winding it up until it starts to get about as hot as you can comfortably hold when it's been running for a while." I'm not sure how accurately those driver current limit numbers are given that they are averaging a pulse output, and the motor current ratings are RMS? Peak? Who knows - the only issue is whether or not they get too hot in practice! Stepper motors are designed to run hot and mine have run using that adjustment technique for a few years now with no apparent ill effect. As for noise - that one's very difficult to judge!